Non-condensable gases almost always cause problems in Rankine cycle power plants, in air conditioning systems, and in other arrangements that utilize condensers. A major problem caused by the presence of non-condensable gases is a reduction in the heat transfer efficiency of various components in a system. That is to say, the presence of non-condensable gases in the working fluid of the system reduces the rate at which heat can be transferred from a heat source to the working fluid in a vaporizer of a Rankine cycle power plant, as well as the rate at which heat can be transferred from vapor to a cooling fluid in a condenser of a Rankine cycle power plant. The deleterious effect noncondensable gases have on the operation of a power plant is illustrated by the example described below of an actual operational system.
Waste heat is applied to a vaporizer of a Rankine cycle power plant utilizing isopentane as the working fluid. The vaporizer vaporizes the working fluid and supplies it to an organic vapor turbine designed to produce 1.5 MW by driving an electric generator. In the turbine, the vapor expands producing work and heat depleted working fluid which is supplied to a condenser. In the condenser, the heat depleted working fluid is condensed into a liquid which is pumped back into the vaporizer to repeat the cycle.
Except during very cold periods of time, the internal pressure at various locations in the power plant described above, including the condenser, will exceed atmospheric pressure. Nevertheless, even under these conditions, experience proves that ambient air leaks into the working fluid through the metal piping, flanges, joints, etc. Apparently, air diffuses through the metal piping and seals even when the pressure inside the system exceeds ambient pressure.
The effect on the power output of a power plant having non-condensable gases in the working fluid is significant. For example, in relatively small systems designed to produce about 1.5 MW, experience has shown more than a 10% decrease in power may result if a constant program of purging noncondensable gases from the system is not carried out, an amount that is significant in terms of the total power output.
The conventional approach to purging non-condensable gases from the condenser of a power plant of the type described is to utilize a vacuum pump arrangement by which fluid (vaporized working fluid and non-condensable gases) in the condenser is admitted to a cooled chamber. The result is a miniature condenser wherein the working fluid condenses and is thus separated from the non-condensable gases which are vented from the chamber before the condensed working fluid is returned to the system.
While this approach is satisfactory in some instances, it is unsatisfactory in many instances because of the power consumption involved, and because of the complex equipment needed to establish and maintain a vacuum. Furthermore, the conventional approach is insensitive to the amount of noncondensable gases in the power plant system requiring continuous operation that, itself, is a disadvantage in many cases. Furthermore, experience proves that extraction of non-condensable gases requires operation of the purging system over long periods of time because the non-condensable gases often are dissolved in the working fluid, and only slowly are released and extracted in the purging system associated with the condenser. Thus, constant operation is often required to ensure removal of these gases. Also, during cooling of the fluid in the cooled chamber, even though a substantial portion of the working fluid is condensed and returned to the system, a large portion of working fluid remains in vapor form and is extracted together with the non-condensable gases during the operation of the vacuum pump. This portion is lost to the system.
It is therefore an object of the present invention to provide a new and improved method of an means for purging non-condensable gases from a condenser which is more efficient than other systems previously known, simpler to maintain control and operate, more sensitive to the actual amount of non-condensable gases in the system, and effective in substantially minimizing the amount of working fluid lost from the system.